giving 4 CPUs in a 4 core machine to Parallels, will make parallels go slower most of the time because OSX doesn't have enough... I'd never give more than half my cores to the virtual machine. WHen I say cores, i mean real cores, not hyperthreading... so if you think you have 8, you don't, you have 4 actual cores.

2 cores will be more than enough, given that most games can handle only one core.
Gaming isn't CPU-stressing anyway, you need GPU power. And this is why you can forget gaming via Parallells, cause it limits your power (in your case, 256MB aren't much anyway) so that both systems get enough.
I try CrossOver or go the BootCamp route.